New upoader

The issue

There are multiple issues with the current method of uploading leaflets:

UX

we ask for one image at a time, in a wizard. This is ok, but the pattern of "upload a new image or finish" is a little confusing

Lambda upload size

Since moving to Lambda, we can only POST 6mb in a single request. This means uploads from modern smartphones won't work, and will give an ugly error (with no sentry reporting).

A solution

We can get around the upload issue, and maybe work on the UX if we move to using JavaScript to upload images directly to S3 (using pre-signed upload keys).

We want to maintain a good fallback for browsers that aren't using JS, so the progressivly enhancing the existing wizard is a quick way of doing this.

Without JS, each image is limited to 6mb, with JS the upload size is unlimited (although we might want to set some bounds on this!)